The use of infra red circuitry to transmit data from one device to another has been used for several years. For example, a television remote control device communicates television commands to a television set via an infrared communication path. The remote control includes a user-input mechanism (e.g., a keypad) which receives input requests from the user. The circuitry of the remote control encodes the command and transmits, via an infrared communication path, the encoded command to the television set. The television set decodes the encoded command to recapture the input command, which the television processes accordingly.
The television remote control, and other similar devices, use an encoding scheme based on a relatively low data transmission rate (i.e., less than two hundred kilobits per second). One such encoding scheme is a modulation technique called amplitude shift keying (ASK). The ASK modulation technique utilizes a presence/absence of a high frequency square wave (e.g., having a frequency of 500 KHz to 2 MHz) at a given rate (e.g., five to ten microseconds) to encode the data. The encoding scheme represents a logic 1 when the high frequency signal is present in a given time interval and a logic 0 when the high frequency signal is not present in the given time interval. As such, the remote control digitally encodes the command by controlling the presence and absence of the high frequency signal within given time intervals. Correspondingly, the television set decodes the ASK modulated signal by detecting the presence and absence of the high frequency signal and assigns the corresponding logic value.
For higher speed infra red data transmissions, such as would be used in many computer applications, a pulse position modulation (PPM) technique is used. Such a technique is proposed in an Infrared Data Association Serial Infrared Physical Layer Length specification (IrDA specification). In general, the IrDA specification defines a four-PPM, 4 Mbps (Megabits per second) infrared modulation technique. The four-PPM modulation technique is based on 500 nanoseconds time chips that are divided into four 125 nSec time slots. Encoding of two bits of data is done by placing a pulse having a 125 nSec pulse width in one of the four time slots. When the pulse is placed in the first time slot, it represents a digital value of 00; when it is placed in the second time slot, it represents a digital value of 01; when it is placed in the third time slot, it represents a digital value of 10, and when it is placed in the fourth time slot, it represents a digital value of 11. Thus, 2 bits of data are transmitted every 500 nSec, or 4 bits per 1 microsecond, which provides the 4 Mbps data rate.
The IrDA standard further requires that a preamble and start flag be used to indicate the start of a data transmission and a stop flag to indicate when data transmission has ended. In addition, the preamble and/or start flag are transmitted periodically to provide synchronization information during the data transmission. The preamble, start flag, and stop flag are encoded and decoded based on a first pulse encoding convention. In particular, the first encoding convention has the preamble, start and stop flags span several time chips, where some time chips include zero pulses, other include one pulse, and still others include 3 pulses. Such an encoding convention is different that the data encoding convention of 4 PPM, which requires a single pulse to be contained with each time chip.
At the 4 Mbps rate, commercial grade light emitting diodes (LED) and light receiving diodes (LRD) are approaching their maximum operating speeds (e.g., commercial grade LEDs and LRDs cost 25 cents or less per part). Typically, the minimum pulse width that a commercial grade light emitting diode can reliably produce given typical bias conditions is approximately 80 nSec. Similarly, the minimum pulse width that a light receiving diode can reliably detect is 80 nSec. In IrDA standard compliant applications, the pulse width of a pulse can vary from 85 nSec to 165 nSec, thus the minimum acceptable pulse width, i.e., pulse duration, is very near the capacity of the LEDs and LRDs.
While the IrDA standard defines a 4 PPM encoding and decoding concept that essentially maximizes the transmission rate of commercial grade LEDs and LRDs, there are many applications that require a data transmission rate greater than the 4 megabits per second. One solution to increasing the data rate is to use higher grade LEDs, and LRDs, but the cost per part is in the range of 5 U.S. dollars per part. Such a cost makes this an impractical solution for commercial applications. Another solution would be to use radio frequency (RF) modulation techniques, however, RF modulators and demodulators are considerably more complex and costly circuits than the 4 PPM encoders and decoders.
Therefore, a need exists for a method and apparatus that achieves higher data transmission rates than the 4 Mbps of IrDA standard, but utilizes commercial grade LEDs and LRDs. In addition, the new method and apparatus should be backward compatible with the components fabricated in compliance with the 4 Mbps IrDA standard.